Not 100% on this, but in passing I thought I saw them advertised on the board of what looked like a frites place yesterday. This was next to the somewhat old fashioned looking Duane Reade and the Taco Bell on the south side of 14th Street just west of Union Square.

Also, upon revisiting, google tells me that Danku (47 W 57th St, New York 10019) should have some version of them (http://twitpic.com/4fjry) but that they likely have more in common with the Trader Joe's variety then the street vendor version.